An optical communication network consists of multiple nodes each of which is controlled by a Network Element (NE). Most optical communication networks incorporate fiber rings or fiber mesh topologies for interconnecting its nodes, both of which can contain closed optical loops at one or more wavelengths within the optical spectrum. In amplified optical systems, the inherent loss of these optical loops is counteracted by the amplifier gain, such that the optical loop may have a net loss that is too low to prevent excessive noise buildup, or a net gain, resulting in a lasing fiber loop. The noise that builds up within such amplified systems is dominated by the Amplified Spontaneous Emission noise resulting in ASE loops.
These low loss loops, or gain loops, can have significant impact on all wavelengths within the fiber, causing partial or complete loss of end-to-end communication due to degraded Signal to Noise Ratio (SNR). Consequently, these loops need to be prevented by deploying appropriate techniques at the NEs. Moreover, if an optical network is susceptible to ASE loop occurrence, due to failures of network devices or elements for instance, the ASE loop susceptibility needs to be identified and eliminated. This is especially problematic in reconfigurable Optical Add Drop Multiplexer (OADM) and Wavelength Selectable Cross Connect (WSXC) environments. Thus, without some prevention mechanism, wavelength rings can be created as a result of simple mis-provisioning events or device failures within reconfigurable OADM and WSXC environments.
Different techniques have been deployed in prior art for avoiding ASE loops. The protection system deployed by one such technique described in U.S. Pat. No. 6,025,941 by Srivastava A. K. et al. issued Feb. 15, 2000 entitled “Stable Wavelength Division Multiplex Ring Network” changes the optical transmission characteristics of the transmission bandwidth of the network in such a way that the loop gain for any wavelength is smaller the network loop loss. Another way to prevent ASE gain loops is to impose one or more optical seams for a wavelength instance. Such a seam prevents the wavelength instance to continue in unspecified directions. For example, an optical seam created by the NE that adds a wavelength instance does not allow the wavelength instance to reach the preceding node. Similarly, an optical seam created by the NE that drops the wavelength instance does not allow the wavelength instance to continue beyond this node. Typically, a detailed network walk is required to ascertain the absence of a loop for a given wavelength instance. These methods either need altering the hardware or deploy a complex algorithm at NEs for loop identification at service creation time. ASE loops can also occur after service creation due to mis-provisioning or device failures. Preferably, the network itself is to be adaptive to events and should not only prevent ASE gain loops under normal operations but can also recognize when the system is susceptible to ASE gain loops under fault conditions. The prior arts discussed earlier do not address the scenario in which a fault has occurred in the system.
Thus there is a need in the field for the development of improved methods and system for avoiding ASE loops in an optical communication network.